Yahoo is set to announce the acquisition of open-source email provider Zimbra, a company that has made headway providing services for other big companies such as Comcast. The news was first reported by AllthingsD, but there was no price mentioned. It’s rumored to be $350 million, according to Techcrunch.

San Mateo, Calif.-based Zimbra’s clients include ISPs and a number of colleges. It was backed with $30.5 million in three rounds from Benchmark Partners, Redpoint Ventures, Accel Partners, Sumitomo and Duff, Ackerman & Goodrich.

The great thing about Zimbra is its flexibility (see past coverage). Its Ajax interface allows nimble switching between calendars, contacts and other features, including mashups so that addresses within emails can pop up as Google maps — although those who have used the service say the user experience is mixed. It kept promising a consumer version, but until now hasn’t delivered it, thus limiting its popularity.

Benchmark Capital and Redpoint Ventures, in particular made a killing. They each owned a quarter of the company after investing about $9 million, the firms told Dow Jones.  They each receive roughly $90 million with this deal, giving them a ten-fold return in four years.

This is the latest result that helps Redpoint redeem itself. The company launched with great fanfare near the top of the bubble in 2000, but didn’t produce anything until about a year ago.

Accel invested later, in a $15 million second round in 2005 and Duff Ackerman & Goodrich, Inventures Group and Presidio STX invested as part of a $14.5 million third round last year.

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